**Core Concept**
The conversion of androgen to oestrogen is a critical process in the growth and development of ovarian follicles, involving a specific enzyme that catalyzes this transformation.

**Why the Correct Answer is Right**
Aromatase is a cytochrome P450 enzyme responsible for converting androgens, such as testosterone and androstenedione, into oestrogens, including estradiol. This process is essential for the growth and maturation of ovarian follicles, as it allows for the production of oestrogens that regulate follicular development and ovulation. Aromatase is highly expressed in granulosa cells of the ovarian follicle, where it plays a key role in the biosynthesis of oestrogens.

**Why Each Wrong Option is Incorrect**
**Option A:** 5 alpha reductase is incorrect because it is involved in the conversion of testosterone to dihydrotestosterone (DHT), a potent androgen, rather than the conversion of androgens to oestrogens.
**Option B:** Desmolase is incorrect because it is an enzyme involved in the biosynthesis of steroids from cholesterol, but it is not directly involved in the conversion of androgens to oestrogens.
**Option C:** Isomerase is incorrect because it is a general term for enzymes that catalyze the rearrangement of molecules, but it is not specific to the conversion of androgens to oestrogens.

**Clinical Pearl / High-Yield Fact**
Aromatase inhibitors are used in fertility treatments to suppress oestrogen production and prevent premature ovulation, making them a useful tool in assisted reproductive technologies.

**✓ Correct Answer: A. Aromatase**